Corticosteroid administration has been associated with a variety of toxicities including osteonecrosis and Cushing's Syndrome, in patients with HIV infection. The prevalence of these toxicities has led to speculation that protease inhibitors impair the cytochrome P450 (CYP) 3A4-mediated metabolism of corticosteroids leading to an increase in their systemic exposure and toxicity. Despite the hypothesized interaction between protease inhibitors and corticosteroids, drug interactions between agents from these pharmacologic classes have not been formally investigated. The purpose of this study is to determine the impact of the HIV protease inhibitor ritonavir on the pharmacokinetics of prednisolone after administration of oral prednisone to healthy volunteers. Ten study subjects received a single 20 mg dose of prednisone before, and after 3 and 14 days of ritonavir exposure at 200 mg twice daily. Blood and urine were collected over 24 hrs after prednisone administration to determine prednisolone pharmacokinetics and renal elimination. Blood was also collected for determination of MDR1 genotype, which may influence prednisolone disposition. Prednisolone concentrations were determined using a validated high performance liquid chromatography method. Non-compartmental methods were used to characterize prednisolone pharmacokinetics before and after ritonavir administration. Pharmacokinetic parameters will be compared using ANOVA with post-hoc testing using the Tukey test. To date, 10 subjects have completed this study and pharmacokinetic data are available for 5 of them. Prednisolone exposure (AUC) was significantly higher in all subjects after 3 and 14 days of ritonavir administration. Geometric mean ratios for prednisolone AUC values pre- and post ritonavir were 1.45 (3 days post ritonavir) and 1.41 (14 days post ritonavir) (P = 0.002 and 0.005, respectively). Prednisolone oral clearance values, after 3 and 14 days of ritonavir, were 74% and 73% of baseline, respectively (P = 0.0004 and 0.0003, respectively). Lastly, the prednisolone half-life was 30% higher after 3 days of ritonavir administration (P = 0.026). In conclusion, an increase in prednisolone exposure and a decrease in clearance were observed in all subjects following ritonavir administration. Inhibition of CYP3A4-mediated prednisolone metabolism is the most plausible mechanism for this pharmacokinetic interaction. The magnitude of the interaction between ritonavir and prednisone (prednisolone) is potentially clinically significant; further study in patients with HIV infection is necessary to determine whether protease inhibitors in combination with corticosteroids are associated with increased corticosteroid-related toxicities (i.e. osteonecrosis).